1. Field of the Invention
The present invention relates to an apparatus for and a method of grinding a crankshaft, more particularly, to an apparatus for and a method of preventing a machining accuracy from deteriorating by restraining a load fluctuation acting on a main spindle when grinding pin portions of a crankshaft.
2. Description of the Related Art
Since a pin portion of a crankshaft used in an engine is rotatably connected to a connecting rod, it is required to accurately machine the pin portion in its radial dimension and roundness.
As disclosed in Japanese Patent Publication (Kokai) No. S54(1979)-71495, it is known such a grinding machine that grinds a pin portion of one crankshaft eccentrically moving around a journal portion as a rotational center, in which two wheel heads are independently advanced and retracted synchronously with a rotation of a main spindle.
In such a conventional grinding machine, the pin portion revolves around the rotational center of the journal portion eccentrically by an eccentric distance between the rotational center of the journal center and a center of the pin portion. Namely, as shown in FIG. 9, a rotational direction of the pin portion relative to a normal component of a grinding resistance changes during a grinding operation either in a case that the pin portion exists at a position represented by (a) in FIG. 9 or in a case that the pin portion exists at a position represented by (b) in FIG. 9. In another words, at the position (a) the grinding resistance acts on the pin portion in a same direction as the rotational direction of the pin portion and however, at the position (b) it acts thereon in a reverse direction relative to the rotational direction of the pin portion. Therefore, there is such a demerit that a grinding accuracy is deteriorated by a load fluctuation acting on the main spindle.
Accordingly, an object of the present invention is to solve the above mentioned problems and is to provide a machining method for grinding pin portions of a crankshaft in which a deterioration is prevented in a machining accuracy of the pin portions by restraining a load fluctuation acting on a main spindle rotating the crankshaft.
Briefly, according to the present invention, two pin portions of one rotating crankshaft having different rotational phase are respectively ground by respective two grinding wheels which are controllably moved synchronously with a rotation of the crankshaft in accordance with pin portion data. In the pin portion data, the two pin portions to be ground simultaneously are memorized as a combination. The two pin portions are different from each other in rotational phase, so that directions of grinding resistance acting on the respective pin portions are also different from each other, Therefore, a load fluctuation acting on a main spindle can be reduced compared with either case that only one pin portion is ground or case that two pin portions having the same rotational phase are simultaneously ground.
Further, a rotational phase difference between the two pin portions in the combination is set as 180xc2x0. In a case that the grinding wheels on the wheel heads rotate in the same condition, the grinding resistances act on the two pin portions by the same amount in positive and negative directions. Accordingly, the grinding resistances can be almost canceled in each other, so that loads acting on the main spindle by the grinding resistances can be almost canceled also, whereby load fluctuation acting thereon can be reduced. Therefore, grinding accuracy (i.e., roundness) on the two pin portions can be improved. Even if a rotational phase difference between the two pin portions in the combination is set as 60xc2x0 or 120xc2x0, the grinding resistances can be reduced in each other, so that loads acting on the main spindle by the grinding resistances can be also reduced.
The load fluctuation acting on the main spindle can be reduced, so that the grinding accuracy (i.e., roundness) on the two pin portions can be improved compared with either case that only one pin portion is ground or case that two pin portions having the same rotational phase are simultaneously ground. The combination of the two pin portions to be simultaneously ground can be freely changed in a condition that the rotational phase difference is set as 60xc2x0 or 120xc2x0. Even if the adjacent two pin portions cannot be simultaneously ground due to the machine construction, the grinding accuracy (i.e., roundness) on the two pin portions can be improved by changing the combination of the two pin portions.
Furthermore, a process table is provided in the memory, in which the combination of the two pin portions and a workpiece No. designating variety of the crankshafts are related, so that a machining process is determined based upon the process table. Therefore, the two pin portions having the different rotational phases can be automatically ground by designating the workpiece No.